Compositions comprising pharmaceutically-active compounds in a liquid or dissolved state are often administered by means of infusion systems, the most simple being an infusion bag with a flexible tube which is connected to an injection needle to be inserted into a vein or similar. The driving force in such a system is gravity. A simple gravity-actuated system is relatively inexact and hence may not be sufficient for the purposes of administering certain active compositions having high requirements regarding exact dosage, both momentarily and during a length of time. The dosage may be very small and the dosing thereof may be extended over a long period of time, which sets a high standard regarding the precision of the delivery system.
There is also a demand for delivery systems, in which the patient can influence or control the dosage of the drug (PCA, Patient Controlled Analgesia), for instance in systems for delivering an anaesthetic or analgesic compositions.
Relatively sophisticated pumps have been developed to meet these standards. However, such pumps, which operate by depressing the infusion tubing, are expensive and complicated to manufacture and to use. Some pumps of this kind are for instance described in U.S. Pat. No. 3,985,133 and U.S. Pat No. 4,396,385.
A pump apparatus comprising a separate pump chamber operable by means of a piezoelectric actuating means and magnetically biassed valve means is known from EP-A-0 398 587. Again, this kind of pump is relatively complicated to manufacture and to use.
For the sake of sterility it also is highly desirable to have a pump which may be designed to be disposable, at least as far as the parts being directly in contact with the active composition are concerned. Pumps utilizing separate, disposable pump parts are for instance disclosed in U.S. Pat. No. 4,140,118, U.S. Pat. No. 4,273,121 and U.S. Pat. No. 4,474,309.
However, in order to be economically viable, the disposable parts have to be inexpensive to manufacture whilst retaining the properties regarding the dosage precision, both momentarily and continuously.
Consequently, one problem to be solved by the invention is to provide a device which whilst being simple to use, is able to dose the active composition in question with extreme exactness, may be manufactured inexpensively as a disposable part and is suitable for use in a PCA-system.
Another problem to be solved by the present invention is also related to the desire for simplicity of use and design of the device, namely the facilitation of the removal of the air initially present in the infusion tubing and in the pump chamber when the pump chamber is being connected into the infusion system. Standard infusion pumps typically require sophisticated detection mechanisms which warn against the presence of air in the infusion tubing. A device, in which it is physically impossible to infuse air into a patient during normal operation, thus eliminating the need for such a detection mechanism, is therefore highly desirable.
Portable infusion pumps normally utilize special cassettes which mostly have to be filled with active compositions(s) at a pharmacy under sterile conditions, which may be costly and inconvenient for the patient. Consequently, there is also a need for portable infusion pumps which may be used with standard prefilled infusion bags which are relatively inexpensive, readily available and which may be delivered in sterile packs.